Electrical properties of compact bone.

Dielectric properties of compact bone tissue have been measured in the wet, i.e., fluid-saturated, state. Comparison of these with other measurements at high relative humidity (RH) shows that the dc conductivity of wet bone is about 100 times larger than that of the high RH sample. Thus, the extrapolation of the high RH results to in vivo situations is not valid. In addition, the results of electrical measurements on dry bone samples cannot be extrapolated to the in vivo state because of the dominance of the fluid-filled pores. The difference in the results for longitudinal, tangential, and radial samples, both in dc resistivity and relaxation time, reflects the difference in connectivity of the pores on bone in these three orientations. Quantitative estimates of the cross-sectional area of connected pores are obtained from measurements on photomicrographs and correlated with dc conductivity of the samples. Further evidence for the dominance of the fluid-filled pores in determining the properties of the tissue comes from the results for bone conductivity g measured as a function of saline conductivity g0. The ratio g/g0 is approximately constant with respect to changes in g0 over a range corresponding to the conductivities of various body fluids. The influence of the dielectric properties in all but destroying the piezoelectrically generated voltage in going from the dry to the wet state is discussed. It is suggested that some mechanism other than the piezoelectric effect (e.g., streaming potentials) must be considered to account for the magnitude and decay time of the electromechanical voltage measured in wet bone. Our studies suggest that fluid transport plays a significant role not only in various aspects of bone metabolism such as mineralization, but also in the electrical, mechanical, and electromechanical properties of bone.